This Non-provisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 2003-299482 filed in Japan on Aug. 25, 2003, the entire contents of which are hereby incorporated by reference.
The present invention relates to a CDMA receiving apparatus, CDMA receiving method, CDMA receiving program and program recording medium and in particular to a mobile CDMA receiving apparatus, CDMA receiving method, CDMA receiving program and a program recording medium in a cellular system using a CDMA scheme.
If a CDMA (code division multiple access) scheme is used for a mobile communication such as cellular phone, the same frequency band can be used on a plurality of channels at the same time. On the transmitter side, a signal to be transmitted is multiplied by a diffusion code which is different for each channel and is then transmitted as so-called diffused signal. Since a code series, the rate of which is tens or hundreds times as high as that of transmission symbol data as the diffusion code, symbol data is diffused in a wider frequency band and transmission is conducted. On the other hand, on the side of CDMA receiving apparatus, transmitted diffused signal is received and is multiplied by a diffusion code (dediffusion code) assigned to the receiving apparatus in synchronization with the receiving timing for conducting a dediffusion for integration over one symbol. Thus, a correlation value is obtained. A large correlation value can be obtained on the receiving apparatus side only if the dediffusion code is the same as the diffusion code on the transmission side. The correlation value of the symbol which is the same as the symbol data which has been transmitted on the transmission side can be obtained as a demodulated symbol.
For demodulation of the received signal, it is necessary to align the timing of the diffusion code by which the diffused signal is multiplied is required to with that of the diffusion code (dediffusion code) which multiplies the received signal on the receiving apparatus side. If the timing is shifted by only one-chip which forms a minimum unit of the diffusion code, the dediffused signal may become a signal which has been multiplied by totally different diffusion code (dediffused code), so that a high correlation value can not be obtained.
Even if multiple paths exist due to reflection and refraction of the electromagnetic waves by buildings or mountains in CDMA scheme, demodulated signal on any one of paths of interest can be properly obtained without being influenced by the other paths if the timing of the diffused signal (dediffused signal) is matched with that of the path.
The distribution of the signal power with respect of the signal power with respect to delay time of each path can be obtained by detecting the signal timing of each path and by plotting the wave shape of the received signal of each timing. Delay profile which represents the timing of each path can be obtained. In other words, received signal of each path is represented as respective independent pulse-shaped wave form of the received signal on the delay profile and is substantially similar to the impulse response characteristics of radio signal propagation path. Therefore, a plurality of, for example, N generators of diffusion code (dediffusing code) and a plurality of, for example, N correlators which determine the correlation between the diffusion code and dediffusion code are provided. The timing of the diffusion code (dediffusion code) is matched with the timings of N paths which are extracted in order of the highest peak value of the wave-forms of received signals from the delay profile. Correlation with the received signals along respective paths is determined by respective correlators for demodulating the received signals. High quality of received signals can be obtained by adding the demodulated signals by using a RAKE combiner which brings the demodulated signals in phase and multiplies them by a proper coefficient.
A technique which detects the positions of timings of the multipath and brings the received signal of each path in phase is disclosed in Japanese Laid-Open Patent Publication No. 2000-312164 titled “spectrum diffusion receiving apparatus,” hereinbelow “Patent reference 1”. The receiving apparatus which is disclosed in Patent reference 1 selects each timing position by a path search which is preliminarily conducted. The received signals of plural paths in each timing position are independently demodulated and are output as demodulated symbols. Timing adjustment is conducted so that the demodulated symbols of respective paths are in phase. A technique for outputting a demodulated and combined symbol adopted.
As is disclosed in Japanese Laid-Open Patent Publication No. 2001-292076 entitled “Multipath detecting circuit,” hereinbelow “Patent reference 2,” when a delay profile for setting the timing position of each path is determined for absorbing path level variations due to phasing, the delay profile is generated by averaging the levels of signals which were received in the past over a predetermined period of time (for example, 100 m seconds) . A path search is conducted based upon the generated delay profile. A demodulated and combined signal is output by RAK combining the demodulated signals of multipath.
The peak power value of the received signal is fluctuated due to the influence of phasing as shown by double headed arrows of solid line in FIG. 3 where the delay profile is generated as shown in FIG. 3. The power components of noise which is mixed with the received signal and the power component of the received signal are fluctuated. Thus, the signal to noise ratio may fall below a threshold level, the signal may not be detected as peak power value which is representative of the path of the received signal. FIG. 3 is a schematic view showing the generation of the delay profile of detecting the path based upon the power level of the received signal. In FIG. 3, paths A, B and C are detected in three timing positions of peak power values a, b and c of the received signal of the delay profile. In order to prevent the peak power value from being unable to recognize as signal path due to the fact that the peak power values a, b and c of the received signal fall below the threshold level or to prevent erroneous detection of the peak power value as signal path due to the fact that the noise level exceeds the threshold level, the levels of the signal including the signals which have been received in past are averaged over a period of time.
The above-mentioned prior art receiving apparatus of the CDMA scheme has a problem which will be described hereafter. The prior art receiving apparatus which determines the timing which represents the peak power value by consecutively averaging the signal levels is not capable of following abrupt change in signal level if the receiving environment abruptly changes so that channel estimating result changes. Specifically, the delay profile used for path search (that is, signal power distribution relative to the delay time of each path), the demodulation symbol of a pilot channel used for channel estimation and cell detection result used for cell selection will be influenced by the result of past measurements since averaging processing of the past measurements is conducted. There is a strong likelihood that demodulation which is adapted to the abruptly changed receiving environment and determination for cell selection can not be conducted. Therefore, if above-mentioned abrupt change in receiving environment occurs, the error rate of the demodulation symbol (received signal) will deteriorate, so that the performance of the receiving apparatus will be lowered.
If the condition of the electromagnetic waves which are being received deteriorates for a prior art receiving apparatus which conducts a demodulation operation independently of the receiving condition, the signal power as well as the noise power would be included in the delay profile used for path search. When the peak of the receiving power (correlation peak) used for determining the signal power is detected, the peak of noise power which is momentarily generated may also be picked up. In a receiving environment in which the electromagnetic waves are thus deteriorating, the peak of the noise power is erroneously included in a modulation path for determining demodulated symbols. The error rate of the demodulated symbol would be deteriorated.
If the condition of the electromagnetic waves which are being received deteriorates for the prior art receiving apparatus which conducts a demodulation operation independently of the receiving condition, it would be difficult to discriminate between the signal power and noise power peaks. Therefore, if a determination is made so that the signal power peak which is not higher than a given threshold is not used for determining a demodulation path which uniquely representing a signal, the path for demodulation for which even low signal power peak is used would be deleted. The error rate of the demodulated symbol is also deteriorated by the amount of the deleted signals.
In a prior art receiving apparatus which conducts a demodulation operation independently of the receiving condition, the length of the constraint when the correlation is calculated for generating the delay profile used for path search is fixed to only a predetermined specific length. Accordingly, if the condition of the radio-waves which are being received deteriorates, the signal power peak can not be discriminated from the noise power peak. Demodulation might be conducted based upon erroneous path. In other words, use of erroneous peak (noise power peak) which is not the single power peak is used for the demodulation symbol. In a prior art receiving apparatus which conducts a demodulation operation independently of the receiving condition, the length of the constraint when the correlation is calculated for generating the delay profile used for path search is fixed to only a predetermined specific length. Accordingly, the length of constraint may be fixed to an excessively long length so that the delay profile can be obtained which is more detailed than the delay profile having information enough to select accurate path. There is a possibility that delay profile which is more detail than required is calculated. Therefore, this may result in that the processing burden becomes heavier and much power is consumed due to the fact that the length of constraint is longer than required.
Furthermore, it is necessary to conduct on a real-time basis a receiving operation which relies upon the wave-form of the received signal like CDMA scheme. Even if data on the wave-forms of the received signal is recorded on a different recording area on each of unit processing lengths, that is processing unit times, recording of the data on the wave-forms of successively and parallely received signals is needed if the data on the wave-forms of the received signals which is recorded on different recording area for each unit processing time is continuously kept for processing which takes an extended period of time. Ultimately, enormous recording area would be necessary.
If reception of stand-by information on a stand-by channel is conducted in a prior art receiving apparatus of CDMA scheme, the power to each of circuits including a radio-unit is turned on in synchronization with the timing of receiving of the stand-by information assigned to receiving apparatus (station) and thereafter a path search is for demodulation relating to stand-by information which was received by the radio unit and then power is turned on until desired stand-by information is demodulated from the detected path information. As a result, the period of time for which the power to each of circuits including the radio unit is turned on is a sum of a period of time for which the stand-by information is received, a period of time taken to path search and a period of time taken to demodulate the stand-by information. This invites an increase in power consumption.
If an operation to receive stand-by information on the stand-by channel is performed, it is necessary to obtain information for detecting channels of peripheral cells depending upon the result of demodulation of the stand-by information, in addition to the reception and demodulation of the stand-by information. In order to shorten the receiving period of time for reducing the power consumption, it is necessary to perform a channel demodulation operation and channel detection operation in a parallel manner. On the other hand, in case of a receiving apparatus in which no recording means (recording buffer) which is capable of consecutively holding data on the wave-forms of received signal on a real-time basis is incorporated therein like a prior art receiving apparatus of CDMA scheme, the receiving apparatus should be configured to include parallel processing systems which simultaneously conduct the channel demodulation operation and channel detection operation. In other words, it is necessary to include two or more processing systems, each comprising a delay profile generating unit (that is, the distribution of the power of signal relative to the delay time of each path) for generating respectively delay profiles from the received signals, a path search unit for detecting a demodulation path based upon the delay profile and a channel demodulating unit for demodulating received signals relating to detected demodulation path and the like. This invites an increase in cost of the receiving apparatus.
If an operation to receive stand-by information on a stand-by channel is performed in a prior art receiving apparatus in which a demodulation operation is performed independently of the receiving condition of the stand-by information, the length of the constraint for determining the delay profile is fixed to only a predetermined specified length as is done in the case of the above-mentioned received signal if an operation to receive stand-by information on a stand-by channel is performed. Therefore, if the receiving condition deteriorates, the possibility to detect the signal power peak related with the stand-by information as well as the power peak caused by noise as demodulation path becomes higher, so that selection of an erroneous path is inevitable on reception of the stand-by path. The error rate of the demodulation result relating to the stand-by information will deteriorate.
In a prior art receiving apparatus which conducts a demodulation operation independently of the receiving condition, the length of the constraint when the delay profile is determined is fixed to only a predetermined specific length if a receiving operation of stand-by information on a stand-by channel is performed. Accordingly, the length of constraint may be fixed to an excessively long length so that the delay profile can be obtained which is more detailed than the delay profile having enough information to select an accurate path. There is a possibility that a delay profile that is more detailed than required is calculated. Therefore, this results in heavier processing burden and greater power consumption due to the fact that the length of constraint is longer than required.
In the prior art receiving apparatus of CDMA scheme, processing units such as delay profile generating unit for generating delay profiles, a path search unit for detecting demodulating paths based upon the delay profiles and a channel demodulating unit for demodulating information on the detected demodulating paths are often all formed of hardware. In particular, the channel demodulating unit which demodulates received signals on multipath on a real-tie basis should include a number of pieces of hardware which is equal to the number of paths. Accordingly, the circuit scale becomes larger and the cost of the receiving, apparatus increases.